Frequency response system



June 2, 1953 E. D. lMMRTHUR FREQUENCY RESPONSE SYSTEM Filed July 26,1949 2 Sheets-Shes?l 1 His Attorney.

` 2 Sheets-Sheet 2 E. D. MCARTHUR FREQUENCY RESPONSE SYSTEM June 2, 1953Filed July 26, 1949 Inventor:

lV/"ibn H is Attor'ny.

h.,` u h t r A C M D w e m E Patented June 2, 1953 FREQUENCY RESPONSESYSTEM Elmer D. McArthur, Schenectady, N. Y., assignor to GeneralElectric Company, a corporation of New York Application July 26, 1949,Serial No. 106,842

(Cl. Z50- 27) 2 Claims.

The present invention relates in general to frequency response systemsand in particular relates to method and apparatus for detectingfrequency or phase changes in electromagnetic energy.

Frequency detection systems are used in many ways. They are used toindicate frequency departures :from a predetermined frequency; they areused to maintain a resonant circuit at a resonant frequency ofpredetermined value; they are used as detectors in frequency modulationreceivers; they are used in automatic frequency control systems.

Frequency detection systems commonly make use of tuned circuits such asresonant combinations of inductance and capacity `and their equivalentsin a manner well known in the art. The frequency sensitivity ofapparatus making use of tuned circuits is dependent upon the sharpnessof the frequency response characteristic of the tuned circuits. Thesharpness of the frequency response characteristic of tuned circuits is,in turn, limited by the power losses, such as the resistive losses, inthe tuned circuit. The power losses can be reduced by judicious designof the tuned circuits; however, there is a physical limitation in theextent power losses in the tuned circuit can be reduced, andconsequently, there is a limitation in the frequency sensitivity of afrequency detection system utilizing conventional tuned circuits.

In certain applications, for instance in radar systems utilizing theDoppler principle, it is desirable to detect frequency variations of afew cycles in several hundred million cycles. The tuned circuit varietyof frequency response circuit is unsuitable for this type of use becauseof the physical limitations aforementioned.

A general object of this invention is to overcome the above-mentionedshortcomings of prior art frequency detectors and to provide a new andimproved method and apparatus for the detection of frequency changes inelectromagnetic energy. By means of the method and apparatus of theinvention, higher sensitivity, for frequency variations than can beachieved with conventional tuned circuits or cavity resonators, isobtained. Another object of the invention is to provide a frequencydiscriminator circuit for use in the ultra high frequency range of theelectromagnetic wave spectrum. A further object of the invention is toprovide frequency response apparatus that is simple to construct and toadjust and which requires a minimum of adjustments.

In general the invention makes use of the fact that a shift of a pointon a standing Wave pattern in a transmission line due to a change infrequency is greater when the aforementioned point is farther from thediscontinuity causing the standing wave pattern. The invention embodiesthis principle in method and apparatus for the detection of frequencyvariations in electromagnetic energy, particularly small Variations infrequency at the high frequency end of the electromagnetic spectrum.

The features of the invention which are novel are pointed out withparticularity in the appended claims. The invention itself together withits further objects and advantages may best be understood by referenceto the following description taken in connection with the accom-`panying drawings in which Fig. 1 is a diagram illustrating theprinciples upon which the present invention is based; Fig. 2 is anelevation view in section of a practical device embodying the invention;Fig. 3 is an elevation view in section of another practical deviceembodying the invention; Fig. 4 is a cross sectional view of apparatusembodying the invention in a frequency standard.

Referring now to Figs. la, lb and 1c there are shown diagrammaticrepresentations of the standing Waves of voltage along a Wavetransmission line when the transmission line is short ycircuited at apoint corresponding to point I on the diagram. Figs. 1c, 1b and 1cprogressively show how the standing Wave expands or shifts as thefrequency of the exciting signal in the transmission line is decreased.It is in part on this standing wave phenomenon upon which the operationof the invention depends. Considering this phenomenon in more detail, ashort circuit on a wave transmission or translating means, such as atransmission line or wave guide, causes the magnitude of the root meansquare voltage of the exciting signal t0 vary periodically along theWave transmission means. A voltage node or minimum point is located atthe short circuit I and every half wave length interval therefrom. Fig.lia shows the shift with respect to the cen- `ter frequency, representedby Fig. 1b, in the standing wave pattern of voltage caused by anincrease in the frequency of the driving voltage. Since the nodes of thestanding Wave pattern of voltage must be a half wavelength apart andsince the first node is at the short circuit a larger number of nodesare formed within a given length of transmission line when the frequencyis increased. Fig. 1c shows the shift in the standing wave pattern whenthe frequency is decreased. At this frequency a smaller number of nodesare formed within a given section of the wave transmission means than atthe center frequency.

Now considering how functionally this phenomenon may be utilized infrequency response systems, consider the points 2 and 3 which for thepurposes of this explanation are fixed on the wave transmission means.At the center frequency represented in Fig 1b, the voltages at points 2and 3 are almost equal. As the frequency is increased the voltage atpoint 2 drops and the voltage at point 3 rises. As the frequencydecreases the voltage at point 2 rises and the voltage at point 3 drops.It is seen that by rectifying the voltages at 2 and 3 and by measuringthe difference of the rectied voltages, a frequency responsecharacteristic, such as shown in Fig 1d can be obtained. At l,the centerfrequency the output from the detector circuit is Zero. As the frequencyis increased, -the output has one polarity and as the frequency is`decreased the loutput .has the opposite polarity.

In yactual practice the standing wave patterns differ from the wavepatterns shown in Eig. 1 because of power dissipation in the physicalcomponents of the wave transmission or translating means. The cusp formof the curve in the vicin- -ity of the minimum voltage points becomes asmooth .curve of gradually changing curvature and in additionthe'rninimum value of the curve does not extend to the zero voltagelaxis but departs slightly from it. However, these differences do notaffect the application of the above- 'described phenomenon for thepurposes of vthe invention.

It is readily appreciated ythat the farther the points 2 and 3 are fromthe short the greater will lbe the voltage change through which `thepoints '2 and 3 pass for a given change of frequency; and thus a systemof high sensitivity for frequency variations could be devised by using along length of line with respect to the wavelength of the energy used todrive the line. It is readily appreciated by :those skilled in the artthat a substantial discontinuity at point I other than a short circuitwill also produce a substantial standing wave pattern on thetransmission line which behaves with frequency in a manner .very similarto the manner described above.

:Referring to Fig. `2, there is shown an embodiment of the inventioncomprising a wave transmission means '4 short circuited by member II,means 5 for coupling energy into the wave transmission means 4, `andprobe means 6 and 'I for detecting the shift in the standing wavepattern along the wave transmission means. The wave transmission kmeans4 comprises a tubular conductor members suitably terminated at each endin conductors ,9 and I0. The member 8 may be rectangular, circular orany other suitable form. Tuning conductor member I.I with'iingers I2 islocated at one end of member 8 and serves to short Acircuit one end ofsaid Wave transmission means so as to reect energy back from member II.Tuning conductor member I 'I is movable along the tubular `conductormember -8. To the tuning conductor member II is connected, as bysoldering, rod member I3 which engages with conductor 9 through vthreadsso that by turning the knurled knob I4 which is suitably connected totherod I3 the tuning conductor member II may be moved along the tubularmember 8 so as yfto mechanically shift a standing wave pattern in-member A8. In place ofthe member I I and its associated adjustingmechanism equivalents well known 'in the art may `be used, for instancechoke pistons. Approximately one-quarter of a wavelength at the centerfrequency of operation from member I Il is located an impedance memberIa having a wave impedance equal to the characteristic or surgeimpedance of member S so that there would be little reflection from thisend of member 8. In general a termination at this end in thecharacteristic impedance of the guide or in any non-reliecting meanswill function satisfactorily.

Energy is coupled into the wave transmission means 4, from a sourcewhich varies in frequency, by means of coupling means 5 which maycomprise a section of transmission line I5 terminated in a loop I6.Coupling means '5 is preferably .located near the end of thetransmission means 4, awayfrom the tuning conductor I I.

A shift in the standing Wave pattern along the conductor member 8 causedby a change in frequency of a signal supplied through loop I6 isdetected by the probe means 6 and 'I which 'are yconnected to thetubular yconductor ymember 8. .Stub ltransmission lines I9 and '2Darezmnunted on the external surface of the tubular conductor member '8registering with 4the vapertures I'I and I8. The stub transmission linesI9 and '20 comprise tubular metallic outer .conductors 2I and .2'2 andcentrally disposed inner Aconductors 23 and 24. Means for `mounting the.stub transmission lines `I 5 and 20 on the external suryface ofconductor comprises retainer members l23 .and 26, which extend `aroundthe apertures I' yand .I8 Yand `are attached to member 8. The frangeportions 2'I and 28 of the stub transmislsion lines I9 and 20 areslideably held between lthe 'retainer :members 2.5 and 26 and the youtersurface of the member 8. The conductors 23 and 24 are internallythreaded to receive the threaded probes 29 and 30, 'to which areconnected the knurled knob members 3I and 32.

Connected kacross `the stub transmission lines I9 and '2U at the points33 and 34 between vthe short circuits 35 and 36 and the openings I'I andI8 are output .circuits comprising 'transmission lines having tubularouter conductors 3T and "38 and centrally -disposed inner conductors 39and 40. rlhe conductors 31 and S38 are connected to the -outerconductors 2| and 22 of the stub transmission line through short sleeves4I and '42 formed integrally with the outer conductors 2I and l22. 'Theconductors39 and 40 may be joined as by soldering to the innerconductors -23 and 24. `The outer conductors 3'! and '38 andthesl'eevesM and :42 vmay Ibe externally threadedand held together by means-of fasteners 43 and 44.

Electromagnetic energy is transferred from the wave 'transmission means4to the stub transmission lines I9 and 20 by means of probes 29 and 30which extend into the space defined by tubular conductor member `8.These probes 29 and '30 comprise a conductive rod which passes through4the longitudinal bores, 45 and '46 in the inner conductors 23 yand 24.The bores 45 and 46 .inconductors 23 and 24 may be threaded if desiredso that the Aend of 'the probes 29 and 30 may be adjusted by means 'ofknurled knobs 3| and 32 attached to the `end of the probes 29 and 30. Inthe `arrangement ldescribed the Apoint of connection of the elements 39and 46 to conductors "23 and 24 is selected so that the distance fromthe points 33 and 34 to the short circuits 35 `and 36 is equal toapproximately a quarter wavelength or an odd multiple of a quarterwavelength of the electromagnetic Wave present in the wave transmissionmeans comprising conductor 8. With this construction the stubtransmission line appears as a high impedance for the high `frequencyelectromagnetic energy so that this energy travels over transmissionlines 39, 4I and 40, 42 to the output circuits. 'Ihe conductors 31, 39and 38. 40 of the output circuits are terminated in shorting members 41and 48 spaced approximately one quarter of a wavelength from the points33 and 34. Integral with the inner conductors 39 and 40 are soldered thecrystal rectifier members 49 and 50. The other ends of crystal rectifiermembers 49 and 50 are connected to conductors 51 and 5B which extendthrough apertures 5I and 52 in members 41 and 48. A by-pass capacitorcomprising respectively plates 41, 59 and dielectric 6I, and plates 48,60 and dielectric 62 bypasses the conductors 51 and 58 to ground. Loadresistors 53 and 54 and lter capacitors 55 and 56 are connected at oneend to the conductor members 39 and 40 and at the Vother end to members31 and 38. The output of the frequency response system is taken betweenconductors 51 and 58. It is appreciated by those skilled in the art thatthere are other ways of connecting rectifier crystals to apparatus suchas shown in Fig. 2 and any of these in general would be suitable for thepurposes of the invention.

A minimum of adjustments are necessary to ready the above device foroperation to detect frequency variations or deviations of the appliedsignal. The general object of the adjustments is to dispose the probes29 and 30 with respect to the standing Wave pattern within the device sothat at the center frequency the voltage output between points 51 and 58is subsantially zero. One way to achieve this general object is, rst, toadjust probes 29 and 39 by means of knob members 3| and 32 so that theyextend the same distance into the member 8. It is desirable not tolocate probes 29 and 30 too far within the member 8 in order to maintainat a minimum any disturbance in the wave propagation properties of theconductor member 8. The probes are next adjusted with respect to eachother until they are approximately one-quarter of a Wavelength apart atthe center frequency of operation of. the device. Finally, the ldeviceis energized by a center frequency signal and by adjusting the shortingelement I I the standing wave pattern in the energized device is movedso that the probes pick up the same magnitude of voltage from adjacentlobes of the standing wave pattern; thus at the center frequency thevoltage between points 51 and 58 is substantially zero. The device isnow ready for operation. If the frequency of a signal supplied to thedevice departs from the center frequency, a unidirectional voltage isdeveloped between points 51 and 58. The polarity and magnitude of thisvoltage depends on the direction and the magnitude of the frequencydeparture.

It is readily appreciated that for large frequency departures the probesare caused to move into the curve portions of the lobes of the standingwave pattern; hence, in this region the output between points 51 and 58would not vary directly with frequency departure. There is thus anoptimum spacing of the probes to make best use of the substantiallylinear, as `distinguished from the curved, portions of the lobes of thestanding wave patterns. This optimum spacing appears to be of the orderof a quarter of a wavelength at the center frequency, if one wishes toretain linearity yat the expense of range.

Variations in the construction of the specic device shown in Fig. 2.readily suggest them- 6 selves. The adjustable element Il could beeliminated and the adjustment function of this element could be assumedby probe assemblies 6 and 1, i. e. probe assemblies 6 and 1 could bemoved together until the condition of null reading is obtained betweenpoints 51 and 58.

It should be noted that the zero setting of the apparatus is independentof any ampltiude variations of the applied voltage.

The performance of the above-described apparatus may be analyzedmathematically and design equations developed, to facilitate itsconstruction and use. It is readily shown by one skilled in the art,that the following relationship exists with respect to the phenomenon onwhich the invention depends:

IVd|=2A cosrsin mrl2 (l) where [Vd] is the absolute magnitude of thedifference in the rectied voltages obtained at points 2 and 3. d is thedistance between points 2 and 3. M is the wavelength of electromagneticenergy at which the voltages at each of the points 2 and 3 with respectto ground are equal; and hence the wavelength at which the rectifiedvolts age between points 2 and 3 is zero. n is the number of wavelengthsbetween the discontinuity l and a point midway between points 2 and 3 atthe wavelength M. A is any wavelength of electromagnetic energy thatproduces a shift in the standing wave pattern so as to produce a netvoltage 'difference between points 2 and 3.

The relationship (1) may be expressed in terms of frequency as well aswavelength. Thus.

sin 'mr In this relationship,

fo=|frequency corresponding to Ao, and f=frequency corresponding to A.

It readily follows from Equation 2 that Vd=l2A cos lrfid-sin mr (3)where is defined by the relationship f =0I fo Vd=2K1rAn cos 1g (4) Byevaluating the constant lc by noting that when 1L1r=30, sin m=.5, it isreadily shown that Vd=6An cos E? (5) Since the factor cos does notchange appreciably as x or the frequency changes, it is readilyappreciated that Vd varies quite linearly with the change in frequencyas shown in Fig. 1, provided the change in frequency is not sufcientlygreat Aso that either points 2 and 3 are in curved regions of thestanding Wave to a close approximation. Hence by substitution of theabove relationship in Equation 3 it is readily apparent that whereVmcvoltage deviation corresponding to frequency deviation mfu and A=aconstant.

It may be further shown from Equation .3 when the deviation 6m is smallthat and hence the frequency sensitivity S of the device would beS=5fl=% cos am...

fm fo From the above equation it `is readily apparent that thesensitivity can be increased at will to very large values determinedbythe magnitude of the factor n by making the structure supporting thestanding wave long. It should be noted that circuit resistivity is not alimiting factor in the above equation which sets forth the factorsaffecting sensitivity. In simple cavity resonators, or tuned circuits ofthe usual type frequency sensitivity is limited by the circuitresistivity.

Referring now to Fig. 3, there is shown another embodiment oftheinvention by means of which the over-all dimensions of the apparatus arekept at a minimum. The conductor 8 and conductor 8a supported byinsulators Sc and 8d v olts Eyie shift (8) comprisea short section ofcoaxial transmission line which is adapted in a manner similar vto themember 8 of Fig. 2 to receive the probe assemblies 6 and 'l and thecouplingl means 5. In order to minimize the space occupied by theapparatus it is desirable to make the section of transmission line 8, 8aquite short. At one end of thesection8, 8a a length of flexible coaxialtransmission .line 62 of low loss shorted at the far end is attached tothe section 8, 3a, through a standard fitting B3 and through endconnector 62a of transmission line 82. At the other end of the section8, 8a a length of lossy coaxial transmission line 64 is connectedthrough a standard fitting 65 and through end connector 64a oftransmission line 64. The section of transmission line 8, 8a. issuitably tapered at bothends so that the characteristic impedance of.the section 8, Ba .matches the characteristic impedance `oi `the`flexible transmission lines 52 and 64. The flexible transmission lineis coiled up to conserve space.

-of different frequencies.

The deviceshown in Fig.'3 functions in amanner similar to the .device ofFig. 2.

The chamber .comprising member y8 in Fig. 2 andcomprising .members 18.and 8a in Fig. 3 can be used as mixing chambers .for mixing signals Incertain applications, it is desirable to yheterodyne or mix twodifferent frequencies. Applicants apparatus is rsuitable for thispurpose. .In addition to the heterodynng function, the apparatus couldat the same time serve'as 'a frequency discriminator or detector. Whenused in this way, the two frequencies preferably may be supplied'to the.chambers through the couplingmeans 5.

The inventionmay also be used asa frequency monitoring apparatus or .asa frequency standard. When used as `a :frequency standard, it isdesirable to control the temperature of the apparatus in order to`maintain the dimensions of the apparatus constant so that a uniformresponse will be obtained for a given frequency. In Fig. 4 is shown theinvention embodied inthe form of a frequency standard. The .apparatusshown in this figure is in general similar to the apparatus shownin'Eig. 2. The section of wave guide between the probe means 6 and landthe short circuited end of the wave guide is rolled into the formvofaspiral to-conserve space and to facilitate the maintenance of thewaveguide at a constant temperature. The block 66 comprising therolled-up wave guide can be constructed in ways well known to thoseskilled in the art. For the purposes of this invention, any of these`ways would in general be suitable. The block .66 is connected to themember '8 by means of fitting 66a. In order to maintain the wave guideand associated apparatus at a constant temperature, Athe apparatus isenclosed in a container 6l, Which may be made of metal or any othersuitable material which will permit maintaining -the apparatus at aconstant temperature. The block 66 andassociated components aremaintained in a fixed position Within the container 61 by means ofsupporting blocks 151),'150, 15d and 15e. The temperature 'of the waveguide and the atmosphere inside the container El is maintained at aconstant temperature by means ofthe heater 68, thermostat 69 and asource of electric power 14, which is connected to the heaterandthermostat by means Aof conductors 12 and "I3 extending through openings'.D and 'll in the container 5l and insulated therefrom by vinsulatingmembers ita and 'l l a. For purposes of adjustment, the top portions ofassemblies S and l extend through the container at the top thereof. Thecoupling means -5 also extends through the container so that energy maybe supplied 'to the apparatus. A direct current voltmeter is connectedbetween points and'58 through insulating members 51a and 58a to indicatefrequency departure.

The apparatus is calibrated by first energizing the heater 6B and`waiting until the temperature of the apparatus has attained a constantvalue which is determined by the setting of the thermostat 69. Astandard signal is then supplied to the apparatus through lthe member 5.The probe assemblies 6 and 'I 'are then adjusted until a null reading isobtained on the meter l5. The apparatus is now ready for operation. Ifan unknown signal is supplied to the apparatus, it is readily determinedby observation of the meter T5 whether it is the same frequency or agreater frequency or a lower frequency .than the standard frequency. Bycalibrating the apparatus on several standard frequencies, the meter 15may be calibrated to indicate not only the departure of the unknownfrequency from a standard frequency but also the extent of thisfrequency departure. Procedures for doing this are readily apparent tothose skilled in the art.

While I have shown and described my invention as applied in and byparticular apparatus, it Will be obvious to those skilled in the artthat changes and modifications may be made Without departing from myinvention and I, therefore, aim in the appended claims to cover all suchchanges and modifications as fall Within the true spirit and scope of myinvention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. Apparatus for producing a signal responsive to frequency deviationsfrom a given center frequency comprising an elongated cavits7 resonator,one end Wall of said resonator comprising a reflective member and theopposite end Wall comprising an absorptive member, means for couplingenergy into said resonator at an intermediate point along its length sothat energy is reflected by said reflective member and absorbed byabsorptive member, a pair of probe means separated along the length ofsaid cavity resonator by substantially one-fourth wave length at saidcenter frequency to provide a continuing differential signal from saidprobe means to detect the shift in magnitude and direction of thestanding Wave pattern in said cavity due to a change in frequency of theenergy coupled into said resonator, said probe means located a distanceequival0 lent to several wave lengths at said center frequency from saidreflective wall to detect large changes in signal for small variationsof frequency.

2. In combination a section of concentric transmission line, a length oflossy flexible coaxial transmission line connected to one end of saidsection of concentric transmission line, a long length of flexiblecoaxial transmission line of low loss shorted at the far end connectedto the other end of said transmission line, means for couplingelectromagnetic energy into said section of concentrictransmissionmline, and means for detectig the sense and the magnitude ofthe shift in the standing Wave pattern along the section of concentrictransmission line comprising a pair of probes separated on said sectionof concentric transmission line by a distance substantially equal to 1Awavelength of the standing wave pattern.

ELMER D. MCARTH'UR.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,151,118 King et al. Mar. 21, 1939 2,413,939 Benware Jan. 7,1947 2,419,208 Franz et al. Apr. 22, 1947 2,420,892 McClellan May 20,1947 2,442,606 Korman June 1, 1948 2,498,548 Howard Feb. 21, 19502,522,563 Blitz Sept. 19, 1950

